The present invention relates to a pressure gauge of the type in which a pressure received by a pressure receiving element is transmitted as displacement or force to a coupling shaft disposed in a gauge body, and more particularly to a pressure gauge in which a liquid is sealed in the body and a temperature compensating diaphragm is provided for lessening the influence of a temperature change of the sealed liquid.
There has heretofore been employed a pressure gauge of the type that receives a pressure to be detected by a pressure receiving element, transmits the pressure as displacement or force of a coupling shaft coupled with the pressure receiving element in a gauge body and detects displacement or force of the coupling shaft by detecting means in the body, thereby to detect the pressure. For properly damping the response of the detecting means to pressure variations and for increasing the stability of the pressure detecting operation, a liquid is sealed in the detector body. Consequently, when ambient temperature or the temperature of a fluid the pressure of which is to be detected varies, the sealed liquid in the body expands or contracts to change the pressure of the sealed liquid, introducing an error in the pressure detection. A solution to this problem in the prior art is to attach a temperature compensating diaphragm to one end face of the gauge body. When the sealed liquid expands or contracts, the temperature compensating diaphragm is displaced correspondingly to cause a variation in the capacity in the gauge body, preventing the pressure of the sealed liquid in the gauge body from undergoing substantial changes.
With the use of such a temperature compensating diaphragm, when the pressure to be detected is high, a sufficient compensation is obtained but when the pressure is low, an error which cannot fully be compensated for by the temperature compensating diaphragm may become nonnegligible in some cases. In view of this, the central portion of the temperature compensating diaphragm is fixed annularly to the end face of the gauge body and the free end of the coupling shaft connected with the pressure receiving element is connected to the center of the annular portion so that the forces which are applied by the pressure of the sealed liquid to the central portion of the diaphragm and the pressure receiving element, respectively, due to a temperature change may equilibrate with each other through the coupling shaft. With such an arrangement, the influence of the pressure variation of the sealed liquid can be made negligibly small when the pressure to be detected is low.
In the pressure gauge, however, the effective area of the pressure receiving element, that is, the overall area of that portion of the pressure receiving element which acts to displace the coupling shaft by the pressure to be detected, is changed in accordance with the range of magnitude of the pressure to be detected. For example, a pressure receiving element of small effective area is employed for the detection of high pressures and a pressure receiving element of large effective area is used for the detection of low pressures. In order to eliminate the influence of the pressure of the sealed liquid to the pressure receiving element by connecting the coupling shaft to the temperature compensating diaphragm, it is necessary that the effective area of that portion of the temperature compensating diaphragm lying inside of the annular fixed portion be equal to the effective area of the pressure receiving element. Accordingly, the radius of the annular fixed portion of the temperature compensating diaphragm must be altered in accordance with the magnitude of the effective area of the pressure receiving element. The central portion of the diaphragm is fixed in a ring-like form to the gauge body, for example, by means of welding. In this case, in order to prevent a hole or holes from being made in the relatively thin diaphragm during welding, the central portion of the diaphragm is formed flat and the flat portion is welded to the gauge body. Accordingly, it is necessary to prepare a plurality of temperature compensating diaphragms respectively corresponding to pressure receiving elements of different effective areas.